Lead-containing lead zirconate titanate is a representative example of piezoelectric materials and is used in a wide variety of piezoelectric elements such as actuators, oscillators, sensors, and filters. Once lead-containing piezoelectric elements are discarded and exposed to acid rain, the lead component in the piezoelectric material may seep into the ground and harm the ecosystem. Accordingly, development and studies of lead-free piezoelectric materials have been actively pursued in order to make lead-free piezoelectric elements.
A most widely studied representative example of lead-free piezoelectric materials is a piezoelectric material that contains potassium niobate. However, in synthesizing a piezoelectric material containing potassium, it is difficult to accurately weigh raw material powders to desired molar ratios due to highly hygroscopic nature of the raw material powders (such as potassium carbonate powder). Moreover, a piezoelectric material that contains potassium niobate (KNbO3) is deliquescent and there have been instances where piezoelectricity of piezoelectric ceramics that contain potassium niobate is degraded with the lapse of time.
NPL 1 reports a piezoelectric material that is free of lead or potassium. This piezoelectric material is a solid solution of sodium niobate (NaNbO3) and barium titanate (BaTiO3) (hereinafter this solid solution is referred to as “NN-BT)). NPL 1 reports that the piezoelectric constant d33 of a piezoelectric ceramic that contained sodium niobate and barium titanate at a 9:1 ratio was 147 pC/N.
PTL 1 offers a method for producing a niobate-based piezoelectric ceramic having a high Curie temperature Tc and good piezoelectric properties. PTL 1 discloses that the piezoelectric constant d33 of a niobate-based piezoelectric ceramic which was a solid solution of NN-BT and strontium titanate (SrTiO3) was 14 to 126 μm/V.